jsp出现空指针，报错处竟然是在

大家请看我这段代码：其本功能是接受表单后更新数据库，然而总在<%这个地方出现空指针异常，不知道是什么回事：我把代码和报错信息贴出来大家看看。
AccUpdate.jsp
<%@ page contentType="text/html;charset=gb2312"%>
<%@ page import="javax.naming.*"%>
<%@ page import="java.io.*"%>
<%@ page import="java.sql.*,javax.sql.*"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>操作结果</title>
</head>
<body>
<h1 align="center">
操作结果
</h1>
<% ----------------------------此行报错！
String accid=request.getParameter("AccId");

String name=request.getParameter("CustName");

float amount=Float.parseFloat(request.getParameter("Savingamount"));//获取参数

InitialContext ctx = new InitialContext();
DataSource ds=(DataSource)ctx.lookup("mysql");
Connection conn=null;
PreparedStatement preStmt=conn.prepareStatement("update account set amount=amount+? where id=? and name=?"); //语句对象

preStmt.setFloat(1,amount);
preStmt.setString(2,accid);
preStmt.setString(3,name);

try{
conn = ds.getConnection();
int ret=preStmt.executeUpdate();
if(ret==0){
%>
<p align="center">
操作失败，输入账号或姓名错误！
</p>
<%
} else {
%>
<p align="center">
存款成功！
</p>
<%
}
} catch (SQLException e) {
e.getMessage();
e.printStackTrace();
} finally {
if (conn != null)
conn.close();
if (preStmt != null)
preStmt.close();
if (ctx != null)
ctx.close();
}
%>
</body>
</html>






报错：Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException: An exception occurred processing JSP page /AccUpdate.jsp at line 15

12: <h1 align="center">
13: 操作结果
14: </h1>
15: <%
16: String accid=request.getParameter("AccId");
17:
18: String name=request.getParameter("CustName");


Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:504)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:415)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:336)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)


root cause

java.lang.NullPointerException
org.apache.jsp.AccUpdate_jsp._jspService(AccUpdate_jsp.java:84)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:373)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:336)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)


note The full stack trace of the root cause is available in the JBossWeb/2.0.1.GA logs.